MONGOID-5882 Isolation state

[jamis]

Some applications will use threads for concurrency. Others will use fibers. Prior to this PR, Mongoid worked fine with threads, but it's internal state could get into odd states when run under fibers.

This PR allows applications to indicate which isolation level they wish to use, and Mongoid's state will be isolated to that scope.

# the default -- inherit the isolation level from `ActiveSupport::IsolatedExecutionState` if possible
Mongoid.isolation_level = :rails

# The following two explicit settings are supported:
Mongoid.isolation_level = :thread # the classic behavior
Mongoid.isolation_level = :fiber  # NEW!

When using the :fiber isolation level, Mongoid's internal state will be inherited from any parent fiber. If you want to make sure a fiber begins with a clean slate, you can wipe the isolation state with Mongoid::Threaded.reset!.

[johnnyshields]

Does :fiber cause weird side effects with embedded callbacks?

[jamis]

Does :fiber cause weird side effects with embedded callbacks?

It shouldn't. That's what most of the tests in this PR are designed to ensure: that nested fibers inherit state from their parent fiber. The entire point of this PR is to make sure Mongoid can be compatible with fiber-based libraries like Falcon.

[johnnyshields]

Cool. 2 more:

1. Is it correct to assume that "fiber" isolation also implies "thread" isolation, as fibers exist on threads (and the "main fiber" of each thread is a separate fiber?) Are there tests for this?
2. Aside from not being on the latest Ruby, is there any reason that a user using Puma (not Falcon) wouldn't want to use :fiber isolation? If no (i.e. fiber is safe) then how about making it the default in Mongoid 10?

[travisbell]

Awesome, thanks @jamis!

Only one comment from me, does it make sense to default the value of Mongoid.isolation_level to the value of ActiveSupport::IsolatedExecutionState.isolation_level? I believe ActiveRecord does, so for continuity, it kind of feels like maybe Mongoid should as well? This way I don't have to remember to set it twice.

P.S. Falcon also assumes this is the standard entry point to make such decisions: https://github.com/socketry/falcon/blob/main/lib/falcon/railtie.rb

[jamis]

@travisbell -- great suggestion. I'll look into it. It's a bit complicated by the fact that we still support Rails 6.x, and the IsolatedExecutionState wasn't introduced until sometime in 7.x, but it's a good idea.

[jamis]

@johnnyshields :

1. Is it correct to assume that "fiber" isolation also implies "thread" isolation, as fibers exist on threads (and the "main fiber" of each thread is a separate fiber?) Are there tests for this?

See https://github.com/mongodb/mongoid/pull/6009/files#diff-fc964a96de10aff651e8e1d4a35745a2a33f357819002b11af3cdacfe3e56979R58-R78

2. Aside from not being on the latest Ruby, is there any reason that a user using Puma (not Falcon) wouldn't want to use :fiber isolation? If no (i.e. fiber is safe) then how about making it the default in Mongoid 10?

It's worth considering. Again, "aside from not being on the latest Ruby" is the sticking point. As long as we support Ruby prior to 3.2, we really can't make :fiber the default.